In the nice setup of my own “un-clouded” PIM (personal information manager), in which DAViCal plays a major role, OS X Lion (10.7) & OS X Mountain Lion (10.8) seemingly can’t handle the groups in the address book (Contacts.app) any longer. – When connecting through CardDAV, the groups just remain empty.
In the respective paragraph 7.1.1 of RFC 6352, which Apple released in August 2011, states:

Description: The CARDDAV:addressbook-home-set property is meant to
allow users to easily find the address book collections owned by
the principal. Typically, users will group all the address book
collections that they own under a common collection. This
property specifies the URL of collections that are either address
book collections or ordinary collections that have child or
descendant address book collections owned by the principal.

… though Contacts.app sets the value of addressbook-home-set to /caldav.php/foobar/ instead of /caldav.php/foobar/addressbook/ in ~/Library/Application\ Support/AddressBook/Sources/XYZ-123456-FOOBAR/Configuration.plist, which is a violation of RFC 6352.